1. Field of the Invention
The present invention relates to an apparatus and method for generating high-power femtosecond pulses, and particularly to a system employing an oscillator, an amplifier, a compressor and a frequency converter to generate femtosecond pulses.
2. Description of the Related Art
Techniques for the generation of short and ultra-short optical pulses in optical fibers have been known for a number of years and have recently been applied in many areas. For example, as disclosed in U.S. Pat. No. 5,530,582 issued to Clark (hereinafter Clark) and as reported by A. Hariharan et al. in "Alexandrite-pumped alexandrite regenerative amplifier for femtosecond pulse amplification", Optics Letters, Vol. 21, pp. 128 (1996) (hereinafter Hariharan), ultra-short optical pulses have been used as injection sources. Injection seeding of high-power amplifiers, as mentioned by Clark and Hariharan, greatly benefits from pulse wavelengths that are in the &lt;1.2 .mu.m range. Similar restrictions exist for many other currently pursued applications of ultra-short pulses, e.g., in THz generation (as reported by W. Denk in "Two-photon excitation in functional biological imaging", Journal of Biomedical Optics, Vol. 1, pp. 296 (1996)) or in confocal microscopy (as reported by van Exter et al. in "Terahertz time-domain spectroscopy of water vapor", Optics Letters, Vol. 14, pp. 1128 (1989)). Such wavelengths can be generated by frequency-doubling of the pulses from an ultrafast erbium fiber oscillator (as suggested by Clark and by L. E. Nelson et al. in "Efficient frequency-doubling of a femtosecond fiber laser", Optics Letters, Vol. 21, pp. 1759 (1996)), i.e., an oscillator generating pulses on the order of 100 fsec, or alternately from fiber oscillator-amplifier systems (as suggested by Hariharan). However, none of these publications teaches that the efficiency of frequency-doubling may be optimized by a restriction of the spectral acceptance bandwidth of the doubling crystal.
Moreover, none of these publications describes that superior performance may also be obtained by implementing oscillator/amplifier designs with nonlinear amplifiers or by implementing oscillator/amplifier designs with nonlinear compressors.
Pulses used in front of the pulse amplifier or compressor do not need to be derived from a fiber oscillator, as described in the above-mentioned publications by Clark and Hariharan. Alternatively, pulses from bulk optics (see Islam et al. in "Broad-bandwidths from frequency-shifting solitons in fibers", Optics Letters, Vol. 14, pp. 379 (1989)) or diode lasers (see Galvanauskas et al. in "Generation of femtosecond optical pulses with nanojoule energy from a diode laser and fiber based system", Appl. Phys. Lett, Vol. 63, pp. 1742 (1993) and Ong et al. in "Subpicosecond soliton compression of gain-switched diode laser pulses using an erbium-doped fiber amplifier", IEEE Journal of Quantum Electronics, Vol. 29, pp. 1701 (1993)) can be used. Note that the systems disclosed by Clark and Hariharan use linear amplifiers and do not suggest that pulses may be derived from a nonlinear amplifier. Further, the systems disclosed by Islam, Galvanauskas and Ong do not use frequency doubling.
In addition, generation of the shortest-possible pulses from oscillator-only type systems, such as those disclosed by Clark and Nelson et al., typically involves complicated cavity designs with relatively high optical losses, which are therefore not very efficient in producing a maximum output power for a given pump power.
With regard to compression schemes in ultra-short optical pulse generating systems, two options exist: using positive dispersion fiber (non-soliton supporting), as disclosed in U.S. Pat. No. 4,913,520 issued to Kafka and by Tamura et al. in "Pulse compression using nonlinear pulse evolution with reduced optical wave breaking in erbium-doped fiber amplifiers with normal group-velocity dispersion", Optics Letters, (1996); or using negative dispersion fiber (soliton supporting) as disclosed by Islam et al. See also, "Peak Power Fluctuations in Optical Pulse Compression", Kafka et al., IEEE Journal of Quantum Elec., Vol. 24, pp. 341 (1988). Though positive dispersion fiber can, in principle, be used for the generation of pulses shorter than 10 fsec, such fibers require additional linear pulse compressors which are prohibitive for a low-cost design. Equally, in compression schemes with positive dispersion fiber, the Raman effect generally cannot be used as the effects of Raman conversion are considered detrimental (see Kafka).
Therefore, pulse compressors based on negative dispersion fiber are preferred, despite the fact that the generated pulse width is typically longer than 10 fsec, since such fiber compressors can be designed not to rely on external linear pulse compressors. In addition, such compressors can be designed to take advantage of the Raman-self-frequency shift in optical fibers, which tends to further broaden the spectrum of the compressed pulses (See Islam et al.).
However, this can lead to the generation of a low-level pedestal in the sought-after compressed pulse, which is deleterious in many applications of ultra-fast optics. On the other hand, the spectral evolution of the Raman pulse is useful, as it allows a certain degree of tunability (see Islam et al.). Such pulse compressors are described herein as soliton-Raman compressors (SRC).
An early system implementation of a SRC in an erbium amplifier fiber was described by K. Kurokawa et al. in "Wavelength-dependent amplification characteristics of femtosecond erbium-doped optical fiber amplifiers", Appl. Phys. Lett., Vol. 58, pp. 2871 (1991). However, in the system disclosed therein, a diode laser provided the "seed" pulses for the SRC.
A further system implementation of a SRC in an erbium amplifier fiber relying on an impractical bulk laser signal and pump sources was reported by I. Y. Kruschev et al. in "Amplification of Femtosecond Pulses in Er.sup.3+ -doped single-mode optical fibers", Electron. Lett., Vol. 26, pp. 456 (1990).
The first implementation of a SRC in an erbium amplifier using a fiber laser as the seed was described by Richardson et al. in "Passive all-fiber source of 30 fs pulses", Electron. Lett., Vol. 28, pp. 778 (1992) and in "Amplification of femtosecond pulses in a passive all-fiber soliton source", Optics Letters, Vol. 17, pp. 1596 (1992). However, the systems disclosed in the publications by Islam et al., Galvanauskas et al., Ong et al., Kafka, Tamura et al., Kurokawa er al., Khrushchev et al. and Richardson et al. did not implement any frequency conversion using a nonlinear amplification system, so as to form a FDW.
Further, the systems disclosed in the publications by Islam et al., Galvanauskas et al., Ong et al., Tamura et al., Kurokawa er al., Khrushchev et al. and Richardson et al. do not provide for any control of the polarization state in the SRC. Currently, SRCs take advantage of the Raman effect in optical fibers, which in turn is dependent on the polarization state of the light in the optical fiber and on the fiber birefringence, as disclosed by Menyak et al. in "Raman effect in birefringent optical fibers", Optics Letters, Vol. 16, pp. 566 (1991). In addition, nonlinear polarization evolution may take place in highly nonlinear SRCs, as reported by Fermann et al. in Optics Letters, Vol. 19, pp. 45 (1994). Hence, a reproducible and stable SRC cannot be constructed without polarization control.
Also, the publications listed in the preceding paragraph do not teach how to maximize the efficiency of SRCs in general and how to maximize the pulse energy of the compressed pulses generated with the SRCs. Since the doubling efficiency obtainable with non-critically phase-matched doubling crystals, such as periodically poled LiNbO.sub.3 (PPLN), is dependent mainly on the pulse energy and not critically dependent on pulse width (when using confocal focussing), pulse energy-maximization is clearly a critical issue.
In the aforementioned publication by Richardson et al., it was suggested that an unspecified control of the mode size of an oscillator fiber and an amplifier fiber can lead to the generation of the shortest possible pulses. However, a maximization of the pulse energy from such a system may in fact produce a higher power at the FDW.
As an alternative to SRC, adiabatic soliton amplification has been discussed for pulse compression (see E. M. Dianov et al., Optics Letters, Vol. 14, pp. 1008 (1989)). In general, the adiabacity condition requires that the gain coefficient .alpha. per soliton period is much smaller than 1. Here, the soliton period of the soliton is defined as L.sub.d .apprxeq.0.5.vertline..beta..sub.2 .vertline./.tau..sup.2, where .tau. is the FWHM (full width half maximum) pulse width of the soliton and .beta..sub.2 is the group-velocity dispersion of the fiber. Alternatively, in adiabatic soliton amplification, the break-up of the fundamental N=1 soliton into an N=2 soliton has to be prevented. As the energy of an N=2 soliton for the same pulse width is 4 times higher than for an N=1 soliton, the gain g per soliton period should be smaller than about 2. As a result, typically amplifier lengths of tens of meters up to km lengths have to be employed, which is not practical. Equally, due to higher-order nonlinear effects in the fiber, the pulse energy after such longer fiber lengths may be lower compared to what is possible with short amplifiers.
As yet another alternative to SRC, femtosecond pulses may be amplified by chirped pulse amplification (CPA) in optical fibers, as disclosed by Minelly et al. in Optics Letters, Vol. 20, pp. 1797 (1995) and in U.S. Pat. No. 5,499,134 issued to Galvanauskas et al. However, typically no pulse-shortening, rather, pulse broadening due to the finite bandwidth of the gain-medium and the employed gratings used for pulse-compression and pulse stretching, is so obtained. Thus, such systems are less useful, unless the pulse energy approaches approximately a few nJ.
In the system described by Minelly et al., a double-clad fiber, i.e. a fiber with a double-step refractive index profile was implemented as the fiber amplifier. Thus, cladding pumping (as disclosed in U.S. Pat. No. 4,829,529 issued to Kafka) was implemented to deliver the pump light into the fiber amplifier. As taught in the publication by Minelly et al., the mode size of the fundamental single mode can be increased in such fibers, while still preserving a high index-raising dopant solvent (Al.sub.2 O.sub.3 in Minelly et al.) concentration. In turn, a high concentration of a dopant solvent can increase the solubility of a dopant (Er.sup.3+ in Minelly et al.), which can lead to a high quantum efficiency for such an amplifier.
However, Minelly et al. do not teach that the performance of such fibers can be optimized for femtosecond pulse amplification by directing the pump light directly into the core, rather than by implementing cladding-pumping.
As an alternative to CPA, linear amplification of short optical pulses can also be considered. Whether an amplifier can be considered linear or not depends on the nonlinear phase delay .PHI..sub.nl incurred by the amplifier. Assuming a linear increase in pulse energy with fiber length in a saturated amplifier and an amplification rate much faster than the adiabaticity condition, the nonlinear phase delay .PHI..sub.nl of a pulse in an amplifier of length L is then given approximately by ##EQU1## where n.sub.2 is the nonlinear refractive index; n.sub.2 =3.2.times.10.sup.-20 W.sup.-1 for silica glass, A is the core area, .lambda. is the signal wavelength and .tau. is the pulse width. Here dispersion-free single-pass amplification was assumed; for double-pass amplification L/2 is replaced by L. Conventional laser amplifiers are typically designed to provide a good pulse quality at a signal wavelength, which implies a design for an amplifier with .PHI..sub.nl &lt;5.
Note that the problem of polarization control in non-polarization maintaining fiber can be minimized by implementing Faraday rotator mirrors (FRMs). However, previous uses of FRMs in fact were limited to linear fiber amplifiers, as disclosed in U.S. Pat. No. 5,303,314 to Duling et al., or ultrafast femtosecond fiber lasers only, as disclosed in the above-mentioned publication by Fermann et al.
All of the above-mentioned articles and patents are incorporated herein by reference.